Spring drive assemblies are known devices that employ resilient means, typically springs, for imparting torque or storing torsional energy. These devices are used in a variety of applications, such as, for example, spring motors (see, for example, U.S. Pat. Nos. 1,258,158, 3,384,321 and 5,590,741), torsional springs for garage doors (see, for example, U.S. Pat. No. 2,481,037), and winches (see, for example, U.S. Pat. No. 5,217,208). Of particular interest herein, are spring assemblies that employ helical torsional springs (hereinafter "helical springs" or "springs").
A helical spring is a substantially cylindrical body having a particular length, diameter and torsional resilience. It is comprised of axially-aligned, constant-radius turns of a flexible material, such as metal wire or rod, which has a certain radial cross-section or "thickness" and a tension modulus of elasticity. Helical springs may be either left- or right-hand wound. Most helical springs are close-wound springs having a body length equal to the wire thickness multiplied by the number of turns plus one. Each spring has two ends which usually extend tangentially from the last turn on either end. Generally, helical springs are mounted around a shaft or arbor, or inside a "cage," so as to be supported at three or more points. Helical springs function within a relatively small space and the interior space defined by the wire turns may contain shafts, adapters, or other springs.
For purposes of discussion herein, a spring is in a "rest" position when unloaded, and is in a "wound" position when loaded. The degree of deflection or winding is measured by its radial displacement from rest. The rate of energy absorption per increment of radial displacement of a spring is represented by its spring constant. Generally, the greater the spring constant, the stiffer the spring is said to be. One equation for the spring constant (k) of a helical spring is as follows: ##EQU1## wherein: M=energy
.theta.=radial displacement PA1 E=tension modulus of elasticity; PA1 D=mean diameter of the spring body; PA1 d=diameter of the spring wire; and PA1 N=number of turns.
A significant problem with helical springs is fatigue. Generally, helical springs are limited in winding to about 15.degree. per turn of the spring. For example, a spring with ten turns (N=10) is limited to about 150.degree. of winding. Therefore, if more than 150.degree. of winding is needed, additional turns are required to avoid fatigue.
Increasing the number of turns on a helical spring, however, frequently is not practical. As mentioned above, helical springs are used commonly in small spaces, such as in winches, which are unable to accommodate longer springs. Although a thinner spring may be used to increase the number of turns without increasing the length, a decrease in thickness exponentially reduces the spring constant as evidenced by Equation (1). Such an exponential reduction cannot be compensated practically by the other spring parameters. Square springs may be used to increase spring cross sectional area without extending length, but they are expensive and generally considered not commercially viable. Therefore, bending fatigue coupled with limitations in space and spring constants result in compromises between a spring's winding capability and its stiffness.
A need therefore exists for a compact spring drive assembly that provides increased winding capability without sacrificing durability or stiffness. The present invention fulfills this need among others.